In many optical printing systems the intensity of a light beam focused on a two-dimensional photosensitive surface is modulated as the beam is moved relative to such surface to provide a two-dimensional output image. Such systems often use an output scanner which may include a gas laser which produces a beam of light at a predetermined wavelength and a deflector such as a rotating polygon mirror which line scans this light beam. The intensity of this laser light beam is modulated by an acoustooptic modulator.
This type of modulator includes a transparent cell which may be made of an acoustooptic material such as glass or TeO.sub.2 crystal and a piezoelectric transducer bonded to the cell. An RF signal, usually in the range of 40-300 MHz, is applied to the transducer. The transducer launches acoustic waves in the cell which produces sonic compression waves that create a diffraction wave grating. This diffraction grating causes a portion of the input laser light beam passing through the cell to be diffracted out of its original path. Amplitude changes of the RF signal cause intensity modulation of the diffracted (first-order) and undiffracted (zero-order) beams. The intensity of the modulated diffracted light beam varies in direct proportion to RF signal amplitude. The modulated diffracted light beam, rather than the undiffracted beam, is utilized, e.g. applied to a deflector which converts the modulated light beam into a line scan.
In some printing applications it is highly desirable that the intensity of the input laser light beam applied to the acoustooptic modulator be kept relatively constant. For example, in certain applications it is desirable that noise, which is variations in such input light beam intensity about a desired constant intensity level, be kept on a very low level such as in the order of less than about 0.1% peak-to-peak intensity of the desired constant intensity level. Controlling the noise, prevents density banding in prints made by the output scanner.
Electrooptic devices such as Pockels devices are used to reduce such noise. A Pockels device includes a polarizer which polarizes a beam of unmodulated laser light and a Pockels cell having spaced transparent electrodes on its end surfaces and an adjustable voltage source for applying a voltage across such electrodes. The voltage establishes an electric field longitudinally along the light transmission path in the cell. A change in voltage, which is proportional to the noise in the input laser beam, causes a change in the electric field. The electric field changes the polarization (azimuth) of the light beam directly as a function of the applied voltage but does not change beam intensity. An analyzer disposed after the Pockels cell converts the polarization changes into light beam intensity changes causing the intensity of the light beam at the output of the analyzer to be at a desired constant intensity level. Such Pockels devices have a disadvantage in that effective transparent electrodes are difficult to make since ideally they should should absorb little light but still be sufficiently conductive to apply a uniform field. Also, Pockels cells are often subject to thermal drift.